1. Field of the Invention
The present invention relates to a gain media edge treatment for reducing or eliminating parasitic oscillations in solid state laser materials. More specifically, it relates to an absorbing material bonded to the roughened edges of a gain medium so as to minimize amplified spontaneous emission (ASE) and at the same time minimize stresses produced within the medium.
2. Description of Related Art
A high peak power solid-state laser, especially one that runs in a pulsed heat capacity mode with solid state gain medium and relatively high gain, typically needs a means of defeating the naturally occurring transverse gain that can lead to losses from amplified spontaneous emission (ASE) and/or to parasitic oscillation. Background information on such deleterious ASE and parasitic oscillation effects can be found in “Fluorescence Amplification and Parasitic Oscillation Limitations in Disk Lasers”, by J. B. Trenholme, NRL Memorandum Rep. 2480, July, 1972; J. E. Swain, et al., J. Appl. Phys., 40, p. 3973 (1969); and J. M. McMahon et al., IEEE J. Quantum Electron. QE-9, p. 992 (1973)).
A technique that has been utilized to suppress ASE and to suppress the onset of parasitic oscillations involves bonding a designed absorbing material to the edges of the gain medium (i.e., an edge cladding). If the index of refraction of the bonded absorbing material substantially matches that of the gain medium, a substantial portion of the ASE is coupled out of the gain media and into the absorbing material before it can build sufficiently to depopulate the excited state and thus reduce or clamp the gain. In general, such claddings include a material that is refractive index matched to the laser gain material and which contains a dopant that absorbs at the laser (ASE) frequency. A number of different materials have been used for cladding, ranging from sprayed-on glass frits to liquids to castings of monolithic glass. (See, for example, G. Dube and N. L. Boling, in Applied Optics, Vol. 13, p. 699 (1974); S. Guch, Jr., in Applied Optics, Vol. 15, p. 1453 (1976); and D. Milam, C. W. Hatcher and J. H. Campbell, in “Platinum Particles in the Nd:doped Disks of Phosphate Glass in the Nova Laser”, in Laser Induced Damage in Optical Materials: 1985: Proceedings of the Boulder Damage Symposium, November 1985, Boulder, Colo.)
Additional background information on edge claddings can be found in, U.S. Pat. No. 4,849,036, incorporated herein by reference in its entirety, titled “Composite Polymer-Glass Edge Cladding For Laser Disks,” issued Jul. 18, 1989 to Powell et al, including the following: “Large neodymium glass laser disks for disk amplifiers such as those used in the Nova laser require an edge cladding which absorbs at 1 micrometer. This cladding prevents edge reflections from causing parasitic oscillations which would otherwise deplete the gain.”
In research by J. E. Murray et al., in “Silicone Rubber Edge Claddings for Laser Disk Amplifiers”, in CLEO 84, Paper No. THF-2 (June, 1984), they report that disk amplifiers can be produced having edge claddings to prevent feedback of ASE. In particular, a room temperature-vulcanized (RTV) silicone rubber is poured about the peripheral edge of the laser disk and plates of filter glass can be embedded in the rubber to absorb ASE. As such, this treatment met most of the requirements of a low-cost, functional edge cladding which can be used on a large laser system comprised of a glass gain media. It is potentially low cost to implement, because the materials are inexpensive, and the process can be applied at room temperature. It is beneficial as an edge cladding, because the cured silicone rubber is water-clear, and its refractive index can be adjusted over the range from about 1.42 to about 1.54, which includes most laser glasses.
However, in crystal and/or ceramic media, the index is usually higher (up to about 1.9) and thus an index of about 1.5 cannot effectively couple out ASE. For normal incidence, the fraction of light reflected in propagating from a material of index n1 to a material of n2 is given by R=((n2-2)(n2+n2))2. For light propagating from a material of index n1 into a material having an index of 1.5, 1.4% of the light is reflected. For steeper angles, the reflection percentage gets substantially higher and at the critical angle all of the light is totally internally reflected. One approach has been to use diffusion bonding of the same crystal material but doped to absorb the ASE due to the gain media. However, because diffusion bonding often requires mating two very flat (<10/λ) surfaces and applying both pressure and heat, it is a very difficult, expensive, and time consuming process with low yield and bonds may have gaps or fail in operation. In addition, since the main crystal and edge cladding are in intimate contact after diffusion bonding, heating of the edge cladding by the ASE introduces stresses back across the bond which can fracture either the crystal or the edge cladding. Such an approach is also time consuming and expensive. Another approach that is beneficial in defeating transverse ASE is to roughen the edges of the slab with bead blasting or other means. This creates very small reflection sites which generates large diffraction losses to the reflected light. However, such a technique, on its own, does not in general sufficiently defeat ASE gain. Still another approach is to cant the edges so as to redirect the ASE normal to the face of the slab. Again, while such a technique is helpful, it decreases the effective area of the slab and is generally not sufficient in itself to defeat ASE gain.